How do I get the special skin from the Potato Sack pack for Portal 2?

This was a bit of a misunderstanding. The 'skin' you receive from the potato sack is the flag for the game you played the most. For example, I got a BIT.TRIP.BEAT flag. This is weird, seeing as Portal 2 supports actual skins, but this is what Valve said.

You have already done what is necessary, now you have only to wait that Valve distributes the skins to all accounts.

I think that is only a matter of time, probably they are busy with a lot of support stuff now. The important is, at the moment, all players are in the same situation and as I know no one have received them yet.